The present invention relates generally to termination of reflected waves or otherwise propagated waves that are unwanted in a network, and in particular to use of partial termination by resistance located at the network hub for a star network.
The simplest network consists of only two sites or nodes. In the case of a two node network, each end of a single transmission line is terminated with a characteristic impedance, which results in no reflections of signals sent in either direction.
This ideal situation is compromised as soon as a third site is added to the network. In the situation of three or more nodes, the intermediate site must not load the transmission line or reflections of transmissions may occur. Thus, for n sites, each of the n sites must be prevented from loading the line, and use of proper termination is necessary to eliminate reflections and other unwanted wave propagation within the network.
Preventing loading and eliminating reflections can be accomplished by using a network configuration in which the transmission line starts at one node and loops through all intermediate sites, terminating at another end, such that the network constitutes essentially a chain of nodes. Such a network is often referred to as a xe2x80x9cringxe2x80x9d network or a xe2x80x9ctoken ringxe2x80x9d network. The ring network, however, is not suitable for many applications, such as wiring in homes and houses and offices, which must connect to a central location.
In contrast to the ring network, which is ideal with regard to loading and reflections, typical networks include wire connections to all sites via a central point, referred to as a xe2x80x9chubxe2x80x9d. This type of network that has a common hub is referred to as a xe2x80x9cstarxe2x80x9d network. The star configuration of nodes simplifies both the wiring of the network and the process of adding or removing a site, but at a great loss in high frequency performance. If the transmission lines are wired in parallel to the hub, from the point of view of any of the transmission lines, the hub appears as a discontinuity of Z0/(n1) ohms (where Z0 is a positive resistance value and n is the number of nodes), which for large networks can result in almost all the energy being reflected from the hub back to the sending station.
Commonly used integrated circuits for signaling, such as those falling under the broad classifications RS-422 and RS-485, for use with networks, are useful with twisted wire connections for a large number of sites. These integrated circuits typically require that no terminating resistance be placed at each site. As a result of this approach, the energy reflected from the hub back to the sending site is almost totally reflected, yet again, from the sending site back to the hub. For a large network having random lengths of connecting wire from the hub to each site, the ensuing cacophony of reflections increases the statistical possibility that the reflections will combine to create a brief reversal of signal polarity and hence a false transition at some site.
Further, in a typical network, transmission lines typically include two wires close together, such as two wires side-by-side, two wires arranged in a twisted pair, or concentric wires (e.g., a coaxial cable for a television). This wiring arrangement therefore constitutes conductors at a fixed distance apart for some considerable length of the conductor, producing a fixed capacitance and inductance per unit length, and thus a corresponding impedance. A characteristic of transmission lines so arranged is that, unlike single wires in a circuit, the wire pairs cannot carry an independent amount of voltage and current: in a single wire circuit, an applied voltage on the wire produces a given current running through the wire, and the voltage and the current typically are totally unrelated (i.e., do not affect each other); however, in a transmission line, the voltage and current are fixed relative to each other in a specific ratio.
Thus, all transmission lines have an impedance associated with them, which is the ratio of the voltage applied to the current that flows in the wire. For example, a transmission line having one volt applied may carry 20 milliamps. The impedance of the cable is the voltage divided by the current. In this example, 1 volt divided by 0.02 amps equals 50 ohms, which is the impedance of the cable.
Fifty ohm cables are commonly used with radios; televisions typically use 75 ohm cables; for twisted pairs such as those used for a 10 Base T network for a computer, the impedance is generally about 100 ohms, but typical impedance is not so precisely defined in this use as for radios and televisions. Category 5 wiring is generally used in businesses for computer networks. This type of wiring includes pairs of wires in which impedance is carefully controlled and repeatable so that each pair of wires has the same impedance.
With a simple network having two nodes connected by a transmission line, in order to minimize impedance and reflection, the transmission line may be terminated such that the resistance equals the impedance of the line. When a wave transmitted from one node reaches the far end of the wire (the receiving end), all of the energy of the wave is received and dissipated in that resistor and, importantly, none of it is reflected. If the resistor at the far end of the line does not match the characteristic impedance of the line, a reflection is produced. The reflection is positive if the resistor value exceeds the impedance of the line and negative if the impedance of the line exceeds the resistance of the resistor.
Thus, matching the resistor to the impedance is simple for a network consisting of two nodes. For example, for a 50 ohm impedance transmission line coaxial cable, a 50 ohm resistor is used at each end of the network; or the transmission line is a twisted pair, 100 ohms is used at each end. Whichever type of transmission line is used, signals travel in both direction between the nodes, reach the respective resistors and are fully dampened.
The appropriate amount of resistance to apply when more than two nodes is involved, however, is much more complicated that the situation with two nodes.
In the xe2x80x9cstarxe2x80x9d network, the central point is referred to as the xe2x80x9chubxe2x80x9d of the network or the xe2x80x9cnetwork hub.xe2x80x9d A problem with the star network is that, once a third site is added, determining the appropriate resistance to place at each node in order to prevent reflection becomes complicated. For example, with a star network having three nodes xe2x80x9ca,xe2x80x9d xe2x80x9cb,xe2x80x9d and xe2x80x9cc,xe2x80x9d and a central hub, the three sites radiate out from a center point, along three lines. If a signal is sent from xe2x80x9caxe2x80x9d to xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d, the signal first proceeds down a spoke to the central hub. At the point of reaching the central hub, the signal must proceed out two spokes to nodes xe2x80x9cbxe2x80x9d and xe2x80x9cc.xe2x80x9d Thus, there is now not a single transmission line of 100 ohms, but two transmission lines. As a result, at the central point, the wave encounters half the impedance of the first spoke: in the example above, the spokes to xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d in parallel produce an impedance for the wave coming down leg xe2x80x9ca,xe2x80x9d of 50 ohms, rather than 100 ohms. The twisted pairs of wires are around 100 ohms impedance each, but two of them wired together in parallel have an impedance of 50 ohms each (R=1/(1/R1+1/R2)=R1R2/(R1+R2)=R1/2, since R1=R2 100 ohms). Similarly, for four spokes, three spokes to which a wave is transmitted produces each have 33 ohms of impedance; with five spokes, each has 25 ohms impedance. This situation becomes even more complicated to the extent that any differences in impedance exist with respect to the different lines.
As a result of the arrangement of the star network, equivalent impedance of the transmission lines tends toward zero as the number of nodes on the network becomes very large. A problem thus exists in the prior art with overcoming the situation in which a signal is sent from xe2x80x9caxe2x80x9d to all the other sites, while preventing, once the signal reaches the central hub, due to the severe impedance mismatch, a large portion of the energy being reflected back to xe2x80x9ca.xe2x80x9d Further, after the wave is reflected from the central hub back to xe2x80x9caxe2x80x9d, if xe2x80x9caxe2x80x9d does not absorb all of the energy of the reflected wave, a second reflection from xe2x80x9caxe2x80x9d occurs. The wave from this second reflection proceeds back to the hub, where yet another reflection occurs, and so on.
A graphical representation of the voltage on the transmission line produced by such reflections, viewed, for example, on an oscilloscope as a function of time, presents a series of oscillations as the wave is traveling down the transmission linexe2x80x94everything is stable until the wave reaches either the hub or node, and then a reflection occurs, with a corresponding new voltage, until that reflection hits the node or hub, again, and so forth, in an oscillating manner. A typical wave so produced is shown in FIG. 1, which presents the voltage at one end of a line as a function of time for a network having 10 100 foot lines.
There exists, therefore, a need for a system, method, and device for constructing a network, which allows simple connection of wires, to solve the problem of reflections in a network. There is a need to solve the problem in such a manner that no particularly special or expensive equipment or expertise is needed to match the impedances and absorb the reflective waves. Thus, there is a need for the problem to be solvable by a single device that is applicable to networks having widely varying arrangement and number of nodes.
The preferred embodiments of the present invention provide systems and devices for a network hub that include a single valued resistor placed in series with each transmission line such that reflected and other unwanted waves in the transmission line are rapidly dampened to prevent interference and errors in transmission.
The preferred embodiments of the present invention also provide methods and systems for determining an appropriate single valued resistor serially placeable within a transmission line in a network such that reflected and other unwanted waves in the transmission line are rapidly dampened to prevent interference and errors in transmission.
The preferred embodiments of the present invention also locate the single valued resistors on a network hub board.
The preferred embodiments of the present invention also provide resistors having resistance of approximately 22 ohms for use with a wide range of impedance transmission lines and signal baud rates.
The present invention solves the problems of the prior art by utilizing a single value resistor in each spoke of the network, in which the resistance is matched appropriately to dampen reflections and other unwanted wave propagations without otherwise significantly impacting the transmitted signal for the network cable used. Instead of trying to match the impedance of each spoke exactly, an embodiment of the present invention involves partially terminating each signal as the signal reaches the hub.
In an embodiment of the present invention, resistors are added in series with each transmission line, so as to partially terminate each reflected signal within the transmission line. An embodiment of the present invention thus uses partial termination to solve the problems of undesired reflective and other waveforms produced in a network line. The partial termination of an embodiment of the present invention uses two series resistors, which are a significant fraction of Zo, the impedance of the transmission line, in series with each line entering the hub. When a site transmits a signal to the hub and then to the other nxe2x88x921 sites on the network, the signal must pass through a partial termination as it enters the hub. Since the impedance of the nxe2x88x921 lines approach zero, the load presented to the incident signal is 2Rs where Rs is the series resistor in each side of the line at the point the line enters the hub.
Increased Rs decreases the reflection coefficient but increases the time to charge the capacitance of all nxe2x88x921 lines. In an embodiment of the present invention, the reflection coefficient of each signal entering the hub is adjusted by varying Rs until a universally applicable Rs is identified for a range of capacitances.
One preferred embodiment of the present invention includes a network hub, comprising a plurality of transmission line connections for connecting a plurality of network nodes to each other, wherein each of the plurality of network nodes is connected to a transmission line, and wherein the transmission line is connectable to at least one of the plurality of transmission line connections; and a plurality of circuit paths for connecting the plurality of transmission line connections, wherein each of the plurality of transmission line connections is connected by a subset of the plurality of circuit paths to every other transmission line connection; wherein each of the plurality of circuit paths includes at least a first resistor coupled to at least a first transmission line connection and at least a second resistor coupled to at least a second transmission line connection and the first resistor is coupled to the second resistor.
Another preferred embodiment of the present invention includes a method for determining a series resistance for partial termination of reflected signals in a network having a network hub and a plurality of nodes, wherein the network hub has a plurality of connections for connecting to the plurality of nodes by a plurality of transmission lines, and wherein a plurality of circuit paths connect each node to every other node, comprising connecting a single value resistor to each of the plurality of connections within each of the plurality of circuit paths, such that each of the plurality of circuit paths includes at least a first resistor coupled to at least a first transmission line connection and at least a second resistor coupled to at least a second transmission line connection and the first resistor is coupled to the second resistor; applying a signal to one of the plurality of connecting lines, the one of the plurality of connecting lines having a transmission voltage; measuring the transmission voltage of the one of the plurality of connecting lines over a predetermined time period; varying the value of the single value resistor and repeating the applying and measuring until the signal is suitably dampened.
Another preferred embodiment of the present invention includes a network for transmitting a signal, the network having a signal dampener, comprising a network hub having a plurality of connection points connected by a plurality of circuit paths, wherein each of the circuit paths connects a connection point to every other connection point; a plurality of nodes connected to the network hub at a plurality of connection points by a plurality of transmission lines; wherein each of the plurality of circuit paths includes a single value dampening resistor coupled to every connection point, and wherein the single value dampening resistor is selected so as suitably dampen the signal.
Additional aspects and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the present disclosure.